Various pad arrangements are adopted in semiconductor chips. For example, there has been known a pad structure in which electrode pads are concentrated along one side of a semiconductor chip (hereinafter, referred as a single-side pad structure). In a semiconductor chip having the single-side pad structure, a chip size can be small, and in addition, a connection area for connection with a circuit base such as a lead frame is set at one place. Therefore, when a plurality of semiconductor chips are stacked stepwise, wire bondability can be maintained. Wire bonding of the electrode pads of such semiconductor chips and inner leads of the lead frame is performed only in an area along one side of each of the semiconductor chips. Therefore, a lead frame on which part of inner leads are routed inside a semiconductor chip mounting area (see JP-A2005-340766 (KOKAI), JP-A 2008-085032 (KOKAI)) is used.
As a chip mounting structure, a structure in which the semiconductor chips are mounted on upper sides of the inner leads routed in the semiconductor chip mounting area (Chip on Lead (COL) structure) is used. The semiconductor chip mounting area on the lead frame is set so as to overlap with an area where the inner leads are disposed. The inner leads routed in the chip mounting area are generally formed in a shape corresponding to the outline of the semiconductor chips mounted on this portion. A resin molding part sealing the semiconductor chips has a shape close to the outline of the semiconductor chips. In other words, the area of the semiconductor chips occupying the resin molding part becomes large.
In the semiconductor chip having the single-side pad structure, it is possible to connect the electrode pads of the semiconductor chip and the inner leads by bonding wires even when the semiconductor chip is smaller than the shape of the inner leads routed in the chip mounting area. If the sizes of the lead frame and the package are made small according to the chip size, a mold and a jig used in a manufacturing process has to be prepared individually. A semiconductor package in which a semiconductor chip smaller than the shape of the inner leads is mounted is in need. In this case, however, the shape of the resin molding part becomes larger than the outline of the semiconductor chip and the area of the semiconductor chip occupying the resin molding part becomes small.
Molding process of resin-sealing a semiconductor chip mounted on a lead frame is generally performed under high temperature of about 180° C. Therefore, in a cooling process to room temperature after the molding, there sometimes occurs warpage in the resin sealing part due to a difference in coefficient of thermal expansion among materials. Since the coefficient of thermal expansion of molding resin is larger than those of the lead frame and the semiconductor chip, a shrinkage amount at the time of cooling from the high temperature of about 180° C. to room temperature is larger in the molding resin. A difference in resin thickness between an upper side and a lower side of the lead frame and the semiconductor chip, if any, results in a larger shrinkage amount of the side with the larger resin thickness than a shrinkage amount of the side with the smaller resin thickness, which causes warpage of the package so that the side with the larger resin thickness is concave.
In a semiconductor package using a lead frame on which some of inner leads are routed in a chip mounting area, when the shape of the inner leads is made to correspond to the outline of a semiconductor chip, an area where the resin thickness differs between an upper side and a lower side of the lead frame (area where the semiconductor chip is not mounted) is small. Therefore, the warpage of the resin sealing part is not very significant. On the other hand, when a semiconductor chip smaller than the shape of inner leads is mounted, an area where resin thickness differs between an upper side and a lower side of a lead frame is large, and thus warpage is likely to occur in a resin sealing part and a warpage amount is also large. Under such circumferences, there has been a demand for preventing the warpage of the resin sealing part.